Quick update:
I just tested to mimic the behavior of the save-button through javascript and it worked.
However when finally pressing the submit-button the user gets a warning along the lines of "Please only use the form navigation buttons for navigating in the survey"

How does the system determine this? Could I just regenerate one of the other hidden input-fields to make the system believe that the user reopened the survey?

In case anyone runs into a need for a "plan-B," I thought I'd mention that took a slightly different approach, and added {SAVE}(Save your work!) to question.pstl in my template, so that the "Resume later" button would appear after each question.

Of course that relies on human behavior, though in realworld use that has also been resulting in the desired outcome on my end.

One question type that presents a particular challenge without some type of autosave or prompting is file-upload, otherwise orphaned files are left on the server when respondents exit before proceeding to the next group.